Optical isolator with improved stabiity

ABSTRACT

An optical isolator is disclosed which comprises a Faraday rotator disposed between a pair of polarization selective elements (e.g., linear polarizers, birefringent wedges, birefringent plates, etc.). Improvement in isolation stability as a function of variations in temperature and/or signal wavelength are achieved in accordance with the teachings of the present invention by utilizing a Faraday device with a rotation θ less than the conventional 45°. A linear reduction in θ, while resulting in a some signal loss, provides a linear increase in both temperature and wavelength stability.

This application is a continuation of application Ser. No. 07/698,436,filed on May 10, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an optical isolator with improvedstability and, more particularly, to the utilization of a Faradayrotator configured to provide improved stability of the opticalisolation with respect to variations in parameters such as temperatureor wavelength.

2. Description of the Prior Art

Reflections in optical systems often generate noise and optical feedbackwhich may degrade the performance of various components, particularlysemiconductor lasers. Therefore, the ability to optically isolate lasersand other sensitive components from these reflections is critical to theperformance of the system. The Faraday effect in magneto-optic materialprovides a unique non-reciprocal device capable of performing theisolation function. In general, a conventional optical isolatorcomprises a 45° Faraday rotator encased in a bias magnet and disposedbetween a pair of polarization selective means (e.g., linear polarizers,birefringent plates, or birefringent wedges) oriented at an angle of 45°to each other. Signals passing through the isolator in the transmitting,forward direction will be essentially unaffected by the polarizationselective means and rotator. However, return reflections will be rotatedsuch that the signal will be essentially blocked from the propagatingback into the signal source (e.g., laser).

The ability of an isolator to block reflected signals is frequentlyexpressed in terms of the "extinction ratio" (ER), where ER (in dB) isdefined as -101 og (I_(R) /I₀, I₀ being defined as the incomingreflected intensity parallel to the output polarizer and I_(R) as theoutgoing reflected intensity from the input polarizer towards the source(e.g., laser). Assuming ideal polarization selective elements (at a 45°angle in relative orientation), the extinction ratio ER can be expressedas -101 og (sin² Θ_(T)), where Θ_(T) is defined as the departure from45° in Faraday rotation. For example, a conventional magneto-opticmaterial such as yttrium iron garnet (YIG) exhibits a temperaturevariation in Faraday rotation of approximately 0.04°/°C. at a wavelengthof 1300 nm (for a conventional 45° Faraday rotator). A material such asa commercially available bismuth-substituted rare earth iron garnet isknown to exhibit an even higher temperature-dependent change in Faradayrotation of approximately 0.06°-0.07°/°C. These changes in rotation as afunction of temperature thus change the rotation imparted on a signalpassing therethrough and, hence, the degree of isolation provided by theFaraday rotator.

U.S. Pat. No. 4,756,687 issued to N. Watanabe et al. on Jul. 12, 1988discloses an exemplary arrangement addressing the temperature stabilityproblem. In particular, Watanabe et al. dislose a cascaded isolatorarrangement comprising a pair of isolators, the first including a 45°Faraday rotator tuned to a wavelength λ₁ slightly less than the systemwavelength λ₀, and the second including a 45° Faraday rotator tuned to awavelength λ₂ slightly greater that λ₀. The temperature-inducedvariation in rotation of the first Faraday rotator is thus substantiallycancelled by the rotation deviation of the second isolator stage.Although the Watanabe et al. arrangement does provide improvement interms of temperature stability, the resulting structure is at leasttwice the size of a conventional isolator and exhibits increased signalloss (as a result of additional components and tuning of the rotatorsaway from the nominal system wavelength).

Another problem with conventional optical isolators is their stabilityas a function of transmitted signal wavelength. In particular, garnetwill exhibit a change in rotation as a function of wavelength. U.S. Pat.No. 4,712,880 issued to M. Shirasaki on Dec. 15, 1987 discloses anisolator arrangement including polarization compensation means forproviding an isolator design which is less sensitive to drift in thesystem wavelength. The Shirasaki arrangement comprises a firstbirefringent wedge plate; a polarization rotation compensator composedof a combination of a half-wave plate and a quarter-wave plate; a 45°Faraday rotator; and a second birefringent wedge. The polarizationrotation compensator may be designed to provide nearly linear isolationover a wavelength range of, for example, 1.3 μm to 1.5 μm. As with theabove Watanabe et al. arrangement, however, the Shirasaki isolatorrequires a number of additional components which adds to the overallcoast and complexity of the resulting design.

Therefore, a need remains in the prior art for an optical isolatorcapable of providing temperature and wavelength stability without undulyincreasing the cost, size or complexity of the resulting arrangement.

SUMMARY OF THE INVENTION

The need remaining in the prior art is addressed by the presentinvention which relates to an optical isolator and, more particularly,to an optical isolator utilizing a Faraday rotator configured to provideimproved stability in parameters such as temperature and wavelength.

In accordance with an exemplary embodiment of the present invention, anoptical isolator is formed which imparts a rotation θ of less then 45°to the polarization state of an optical signal passing therethrough. Inparticular, an inventive optical isolator may include a magneto-opticelement (hereinafter referred to as a Faraday rotator) disposed betweena pair of polarization selective elements. The reduction in rotation(Δθ) from 45°, while slightly reducing the optical power of the outputsignal, has been found to increase both the temperature and wavelengthstability of the isolator. In particular, there exists a linearrelationship between the reduction of the rotation (i.e., Δθ) and thereduction in the variation of the Faraday rotation with changes inwavelength (Δλ) and temperature (ΔT).

In a preferred embodiment of the present invention, the reduction inrotation is accomplished by providing a Faraday rotator with a reducedthickness t as compared to a conventional 45° rotator, since rotation isdirectly proportional to material thickness for a given composition. Foran exemplary arrangement of the present invention, a Faraday rotator maybe configured to provide a rotation of approximately 10% less than thenominal 45°, (in the range of, for example, θ=40°-42°), thus resultingin an increase in temperature and wavelength stability in the reverse(isolation) direction of approximately 10%. With the pair ofpolarization selective elements disposed to one another at an angle αsuch that θ+α≅90°, the isolator of the present invention will be capableof essentially blocking any reflected signals incident thereon. The costof increasing the temperature and wavelength stability is a slightreduction in the transmission of the isolator in the forward(transmitting) direction.

Alternatively, reduction in rotation may be accomplished by reducing theintensity of the magnetic field, B, utilized to induce the Faradayeffect in the magneto-optical material. As with the physical materialreduction described above, a linear reduction in the strength of theapplied field will yield a similar improvement in both temperature andwavelength stability.

In one embodiment of the present invention, the requirements of outputpower and isolation may be altered such that the isolator experiencesimproved output power, at the cost of a slight reduction in isolationperformance. For this embodiment, the pair of polarization selectiveelements may be disposed at an angle of α to one another such that α≅θ,where the close α is to θ, the more the output power level will approach100% (at the cost of decreased isolation). Accordingly, there exists atrade-off between the requirements of optical power and isolation whichmay be considered on a case-by-case basis while utilizing the isolatorconfiguration of the present invention and enjoying the increasedtemperature and wavelength stability.

An advantage of the present invention is that the improvement intemperature and wavelength stability may be provided by generating areduced Faraday rotation without modifying any of the components withinthe isolator arrangement. Accordingly, the isolator design of thepresent invention may be utilized with a variety of differentpolarization selective means including, but not limited to, linearpolarizers, birefringent wedges, birefringent plates, half-wave plates,quarter-wave plates, or any combination of the above. Further, theability to form a rotation less than 45° is not dependent upon the typeof magneto-optic material utilized, since the rotation, as discussedbelow, may be controlled by changing the thickness of themagneto-optical material, applied magnetic field, or both.

Other and further advantages of the present invention will becomeapparent during the course of the following discussion and by referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

Referring now to the drawings,

FIG. 1 illustrates an exemplary optical isolator formed in accordancewith the present invention, where the configuration of the Faradayrotator (either reduced material thickness, applied magnetic field, orboth) is chosen such that a rotation θ of less than 45° is achieved;

FIG. 2 is a graph illustrating isolation as a function of changes intemperature, for both a conventional 45° Faraday rotator and a 40.5°Faraday rotator formed in accordance with the present invention; and

FIG. 3 is a graph illustrating isolation as a function of changes insignal wavelength, for both a conventional 45° Faraday rotator and a40.5° Faraday rotator formed in accordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates, in simplified form, an exemplary optical isolator 10formed in accordance with the present invention. As shown, isolator 10comprises a Faraday rotator 12 (for example, a magneto-optic materialsuch as yttrium iron garnet (YIG) or bismuth-substituted rare earth irongarnet) which comprises a predetermined thickness t and/or an appliedmagnetic field intensity B capable of providing a rotation (e.g.,clockwise) through rotator 12 of less than 45°. Faraday rotator 12 isillustrated as disposed between a pair of polarization selectiveelements, which for this particular embodiment are illustrated as a pairof linear polarizers 14,16. As shown, polarizer 16 is oriented at anangle α with respect to polarizer 14. As mentioned above, the values forθ and α may be chosen such that θ+α≅90°. to provide maximum opticalisolation in the reverse direction. Alternatively, choosing θ≅α willprovide maximum output power. In either case, θ is defined to be lessthan 45° in accordance with the teachings of the present invention and αmay comprise any suitable value for the operation of the isolator. It isto be understood that the illustration of linear polarizers is exemplaryonly and various other polarization selective elements including, butnot limited to, birefringent wedges, birefringent plates, half-waveplates, quarter-wave plates, or any combination of components capable ofproviding the desired polarization selective function, may be used inassociation with the Faraday rotator design of the present invention.

In operation, a transmitted signal I_(T) is coupled (by a first lens 18,for example) into isolator 10 along optical axis OA. As shown in FIG. 1,isolator 10 is oriented so that first polarizer 14 is aligned with theplane of polarization of signal I_(T) such that signal I_(T) passestherethrough essentially unimpeded. Signal I_(T) next encounters Faradayrotator 12, where the polarization state of signal I_(T) is rotated (forexample, clockwise as shown in FIG. 1) as it propagates through thethickness t of Faraday rotator 12. Upon exiting Faraday rotator 12, thepolarization state of signal I_(T) has rotated θ°, as shown in FIG. 1,where θ (as controlled by the thickness t and applied magnetic field B)is chosen to be less than 45°. Rotated signal I_(T) next passes throughsecond polarizer 16 (disposed at an angle α with respect to optical axisOA). The output from second polarizer 16, denoted I'_(T) is then coupled(by a second lens 20, for example) into the output of isolator 10.Assuming that θ +α≅90°, a slight loss of signal will occur at the outputof isolator 10, since α is not equal to θ. In particular, thetransmitted output power I'_(T) may be defined by:

    I'.sub.T =I.sub.0 κcos.sup.2 [α-θ],

wherein I₀ is defined as the incident power of signal I_(T) at the inputto first polarizer 14 and κ is defined as a loss term due to absorption,scattering and Fresnel losses. Assuming θ is chosen as 42° and θ+α≅90°,then α is approximately 48°. The cos² expression in eq.(1) results in avalue of approximately 0.9891, representing a loss in output signalpower of approximately 2%. Alternatively, assuming that θ≅α, the samecos² term results in a value of 1, representing approximately 100%transmission of the signal through isolator 10. As mentioned above, anysuitable value for α may be utilized, the two values of α chosen aboveunderstood as being exemplary only.

In accordance with isolation technology, the function of arrangement 10is to prevent reflected signals from re-entering optically sensitivecomponents located "upstream" of the signal path, such as a transmittinglaser (not shown). That is, a reflected signal I_(R) (which in the worstcase would be aligned with the polarization state of second polarizer16) needs to be blocked from re-entering optically sensitive devices. Asshown in FIG. 1, a reflected signal I_(R) may be coupled by lens 20 intoisolator 10 and propagate through second polarizer 16 essentiallyunimpeded (worst case), assuming alignment of the polarization state ofsignal I_(R) with the orientation of second polarizer 16. Reflectedsignal I_(R) next propagates through the thickness t of Faraday rotator12. Since a Faraday rotator (regardless of its thickness) is anon-reciprocal device, the polarization state of signal I_(R) willexperience the same clockwise rotation (of θ°) as transmitted signalI_(T). Assuming that θ+α≅90°, signal I_(R) will exit Faraday rotator 12essentially perpendicular to the polarization plane of first polarizer14, as shown in FIG. 1. Thus, reflected signal I_(R) will be essentiallyprevented from passing through first polarizer 14 and entering anydownstream component. Alternatively, assuming that α≅θ, signal I_(R)will exit rotator 12 at an angle 2θ with respect to the polarizationstate of first polarizer 14 (where 20<90°). Therefore, a slight returnsignal (.sup.˜ 2-3%) will be coupled into the signal path, which may bedescribed as a reduction in isolation.

Conventional isolators are known exhibit a degree of dependence on boththe ambient temperature (T) and the wavelength (λ) of the signal passingtherethrough. For example, the temperature coefficient of a 45° Faradayrotator comprising YIG (intended for operation with an optical signal ata wavelength of approximately 1300 or 1500 nm) is known to be about0.04°/°C., and those using a bismuth-substituted rare earth iron are inthe range of 0.06°-0.07°/°C. In conventional arrangements, a 45° Faradayrotator is designed to exhibit a zero-valued temperature variation at aparticular temperature and wavelength (e.g., room temperature and λ₀ of1.5 μm). FIG. 2 illustrates, in curve A, the effect of temperaturevariation on the isolation function of a conventional 45° Faradayrotator. As shown, the Faraday rotator exhibits maximum isolation (>40dB) at the designed temperature (e.g., room temperature). As thetemperature either increases above or decreases below this value, theisolation diminishes in the manner illustrated in curve A of FIG. 2.FIG. 3 illustrates, in curve A, a similar graph for the effect ofwavelength variation on isolation. Here, as the system wavelength driftsaway from the nominal value (e.g., 1.5 μm), the isolation decreases.

As will be discussed in detail below, an improvement in both temperatureand wavelength stability may be achieved in accordance with theteachings of the present invention by reducing the rotation θ of Faradayrotator 12. In particular, the rotation θ may be expressed as

    θ=R(λ,T,B)×t

where R is defined as the specific rotation of a particularmagneto-optic material (and is dependent upon λ, T and B), and t is thephysical thickness of the material. In accordance with the teachings ofthe present invention, it is desired to minimize the change in reflectedsignal I_(R) for variations in either λ or T. In particular, reflectedsignal I_(R) can be expressed as:

    I.sub.R =I.sub.0 κ[cos.sup.2 (θ+α)+β],

where I₀ is defined as the incident powder of reflected I_(R) at theoutput of isolator 10. The term κ is defined as a loss term due toabsorption, scattering and Fresnel loss, and β is defined as a limitingvalue on isolation related to the limited extinction ratio of thepolarization selective means, scattering and/or birefringence losses. Asmentioned above, assuming that the polarization state of I₀ is alignedwith the orientation of second polarizer 16 provides the worst caseanalysis. Calculating the derivative of equation (3) with respect to theparameters λ and T thus provides the analysis required to minimize thevariations. In particular, the derivative of I_(R) with respect to λ maybe expressed as: ##EQU1## Similarly, the derivative with respect to Tcan be expressed as: ##EQU2## Substituting from equation (2), the abovederivatives can be rewritten as: ##EQU3## Since the quantity R(λ,T,B)and its partial derivatives are defined by the material used to form therotator (and, therefore, cannot be controlled), the only way to minimizethe above variations in reflected signal I_(R) is to reduce θ, theactual rotation of the wave through the Faraday rotator.

In the implementation of the present invention, the reduction in θ maybe achieved by reducing either the thickness t of the rotator material,or the intensity of the applied magnetic field B (where θ=R(λ,T,B)t).For example, a 10% reduction in rotation θ (i.e., from 45.0° to 40.5°)has been found to yield an approximate 10% increase in both wavelengthand temperature stability. FIG. 2 illustrates, in curve B, theimprovement in isolation as a function of the increase in temperaturestability when utilizing a Faraday rotator of θ<45° (in particular, forθ=40.5°) in accordance with the teachings of the present invention.Similarly, FIG. 3 illustrates, in curve B, the improvement in isolationas a function of the increase in wavelength stability when utilizing anexemplary Faraday rotator formed in accordance with the teachings of thepresent invention.

It is to be understood that the above-described embodiments of thepresent invention are exemplary only and various modifications may bemade by those skilled in the art of optical isolation. For example, theFaraday rotator design of the present invention may be utilized inconjunction with virtually any components capable of providing opticalisolation, as long as the rotation provided by the Faraday rotator ismaintained less than 45°.

We claim:
 1. An optical isolator comprisinga first polarizationselective element; a second polarization selective element disposed atan angle α with respect to said first polarization selective element;Faraday rotation means disposed in an optical signal path between saidfirst and second elements characterized in that the Faraday rotationmeans comprises a magnetic field source for generating a magnetic fieldB of a predetermined intensity; and magneto-optic means comprising apredetermined thickness t and coupled to the magnetic field source,wherein either one or both of said predetermined field intensity andsaid thickness t are chosen such that said Faraday rotation meansexhibits a predetermined stability of isolation with respect to changesin temperature or wavelength by providing a rotation θ of less than 45°to an optical signal passing therethrough.
 2. An optical isolator asdefined in claim 1 wherein the Faraday rotation means comprisesmagneto-optic means which has a predetermined thickness t capable ofproviding the desired rotation θ of less than 45°.
 3. An opticalisolator as defined in claim 1 wherein the predetermined magnetic fieldintensity is chosen to generate a rotation of less than 45° within themagneto-optical material.
 4. An optical isolator as defined in claim 1wherein wherein the predetermined thickness t and the magnetic fieldintensity B are chosen to provide a rotation θ of less than 45°.
 5. Anoptical isolator as defined in claim 1 wherein the Faraday rotatorincludes a rare earth iron garnet magneto-optical material.
 6. Anoptical isolator as defined in claim 5 wherein the rare earth irongarnet material comprises yttrium iron garnet.
 7. An optical oscillatoras defined in claim 5 wherein the rare earth iron garnet materialcomprises bismuth-substituted rare earth iron garnet.
 8. An opticalisolator as defined in claim 1 wherein α is chosen such that θ+α≦90°. 9.An optical isolator as defined in claim 8 wherein α+θ≅90°.
 10. Anoptical isolator as defined in claim 8 wherein α≅θ.
 11. An opticalisolator as defined in claim 1 wherein θ is within the range ofaproximately 40°-43°.
 12. An optical isolator as defined in claim 1wherein the first and second polarization selective elements are chosenfrom the group consisting of linear polarizers, birefringent wedges,birefringent plates, half-wave plates, quarter-wave plates.
 13. Anoptical isolator as defined in claim 12 wherein the first and secondpolarization selective elements comprise first and second linearpolarizers.
 14. An optical isolator as defined in claim 12 wherein thefirst and second polarization selective elements comprise first andsecond birefringent wedges.
 15. An optical isolator including a Faradayrotatorcharacterized in that the Faraday rotator comprises a magneticfield source for generating a magnetic field B of a predeterminedintensity; and magneto-optic means, comprising a predetermined thicknesst, coupled to the magnetic field source wherein either one or both ofthe predetermined field intensity B and predetermined thickness t arechosen such that said Faraday rotator exhibits a predetermined stabilityof isolation with respect to changes in temperature or wavelength byproviding a rotation θ of less than 45° to an optical signal passingtherethrough.
 16. An optical isolator as defined in claim 15 wherein theFaraday rotator comprises a magneto-optic material having apredetermined thickness t determined to provide a rotation θ less than45°.
 17. An optical isolator as defined in claim 15 wherein thepredetermined magnetic field intensity is chosen to provide the desiredrotation of less than 45° within the magneto-optic means.
 18. An opticalisolator as defined in claim 15 wherein the predetermined thickness andand magnetic field intensity are chosen to provide a rotation θ of lessthan 45°.
 19. An optical isolator as defined in claim 15 wherein theFaraday rotator comprises a rare earth iron garnet magneto-opticmaterial.
 20. An optical isolator as defined in claim 19 wherein therare earth iron garnet comprises yttrium iron garnet (YIG).
 21. Anoptical isolator as defined in claim 20 wherein the rare earth irongarnet comprises bismuth-substituted rare earth iron garnet.